Expanded metal filters

ABSTRACT

Expanded metal sheets are used to produce tubular filters. The expanded metal sheet has a multiplicity of rows of openings arranged to reduce nesting when the sheet is rolled on itself. In particular, the pitch between the rows of openings, the sizes of the openings, or both the pitch between the rows of openings and the sizes of the openings are varied to reduce nesting when the expanded metal sheet is rolled on itself. The filters can include external circumferential grooves, rounded corners produced by a point loading process, textured surfaces between openings, and/or torturous internal paths produced by non-perforated areas of an expanded metal sheet. The expanded metal sheet can be composed of carbon steel coated with a material having a higher heat conductivity, e.g., tin. Among other things, the filters can be used in automobile airbag inflators to capture slag and absorb heat produced by the inflator&#39;s explosive charge.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International ApplicationNo. PCT/US2007/078971 filed Sep. 20, 2007, which claims the benefitunder 35 USC §119(e) of U.S. Provisional Application No. 60/846,381filed Sep. 21, 2006 and U.S. Provisional Application No. 60/851,719filed Oct. 14, 2006, and which was published in English asWO/2008/036788 on Mar. 27, 2008. The contents of InternationalApplication No. PCT/US2007/078971 and U.S. Provisional Applications Nos.60/846,381 and 60/851,719 are hereby incorporated herein by reference intheir entireties.

FIELD

This invention relates to expanded metal sheets having variable sizedopenings, variable opening pitches, or both variable sized openings andvariable opening pitches so that the openings are less likely to alignand nest when the sheet is rolled on itself, methods for making thesheets, filters made from the sheets, and methods for making thefilters.

BACKGROUND

A. Expanded Metal Sheets

Expanded metal sheets have found a variety of uses, from mats used forfighting fires to filters for automobile airbag inflators. They can bemade in a variety of ways. For example, an expanded metal sheet can bemade by taking a sheet of metal, puncturing the sheet to produce amultiplicity of slits, and pulling the sheet perpendicular to thedirection of the slit to elongate the slit and provide an opening in thesheet. Another common method for making an expanded metal sheet is bypiercing and cold forming openings, which are often called “diamonds”because of their final shape. The final length of the sheet, with theaccompanying holes, is longer than the original and so it is expanded,as well as the openings formed being expanded.

Thus, although the details will vary depending on the specific process,expanded metal sheets are typically made by using a row of teeth or bitsin a punch to produce perforations in the sheet. The side of the sheetfacing the punch will have an indentation around the perforation, andthe reverse side of the sheet will have a corresponding raised portion,a burr, around the perforation. The regularity of the perforationsallows nesting of the perforations when the sheet is stacked, curled,rolled or otherwise placed in overlying relation, and the presence ofburrs can lock the structure in a nested configuration. The burraccompanying each perforation also creates an area of increased frictionso that the expanded metal sheet may not slide easily, and especiallynot when in contact with itself when curled or wrapped around a similarsheet.

B. Filters for Automotive Airbag Inflators

Filters for automotive airbag inflators need to satisfy a number ofdemanding criteria. Such filters serve to capture the extensive debristhat is generated when an airbag's explosive charge ignites. This debriscan damage the airbag and if released from the airbag can injureoccupants of the vehicle in which the airbag deployed. In addition, thedebris is often chemically harmful to humans.

To control this debris, filters for automotive airbag inflators need tobe highly effective in their filtering function. Yet, they must alsoallow the gas generated by the explosive charge to quickly reach andinflate the airbag. That is, the filters cannot generate excessivelevels of backpressure. Moreover, the filters need to satisfy theseconflicting criteria, i.e., effective filtering with low backpressure,in the midst of a strong explosion. Besides these criteria, the filteralso serves as a diffuser to attain a more even flow of the expandinggases entering the airbag and as a heat sink to help reduce thetemperature of the gases so that they will not harm the airbag or theperson being protected by the airbag.

In addition to these considerations, cost is always an issue for amass-produced item, especially one used in the automotive field.Consequently, there have been continuing and extensive efforts toproduce low cost and yet highly effective filters for airbag inflators.

SUMMARY

In accordance with a first aspect, a tubular filter is disclosedcomprising an expanded metal sheet having a multiplicity of rows ofopenings arranged to reduce nesting when the sheet is rolled on itself,the sheet rolled into a tube, and welded to fix the tubular orientation,wherein:

(a) the expanded metal sheet is formed from a larger sheet of metalhaving a width and an axis;

(b) the expanded metal sheet has long edges formed by the larger sheetof metal and short edges where it has been cut as a portion of thelarger sheet;

(c) the openings are formed by forming slits in the larger sheet ofmetal and stretching the slits in the direction of the axis; and

(d) the pitch between the rows of openings, the sizes of the openings,or both the pitch between the rows of openings and the sizes of theopenings are varied to reduce nesting when the expanded metal sheet isrolled on itself (e.g., the pitch between rows of openings is varied asa function of the circumference defined by a given portion of theexpanded metal sheet in the rolled up filter so that radially adjacentopenings do not nest).

In accordance with a second aspect, a method of making a filter isdisclosed that includes:

(a) providing a sheet of metal having a width and an axis;

(b) forming slits in the sheet and stretching the slits in the directionof the sheet's axis to form a multiplicity of rows of openings;

(c) cutting a smaller sheet from the sheet produced in step (b);

(d) rolling the smaller sheet of step (c) on itself to form a tube; and

(e) securing the tube of step (d) with a weld;

wherein in step (b), the multiplicity of rows of openings are arrangedto reduce nesting when the smaller sheet is rolled on itself to form thetube (e.g., the pitch between the rows of openings, the sizes of theopenings, or both the pitch between the rows of openings and the sizesof the openings are varied to reduce nesting when the smaller sheet isrolled on itself to form the tube).

In accordance with a third aspect, a tubular filter is disclosed thatcomprises expanded metal that has been rolled on itself to form amulti-layered tube wherein the expanded metal comprises a multiplicityof openings and includes at least one section that is free of openings,said section producing circumferential flow of gas within the filter.

In accordance with a fourth aspect, a tubular filter is disclosed thatcomprises expanded metal that has been rolled on itself to form amulti-layered tube having a substantially cylindrical outer surfacewherein the expanded metal comprises a multiplicity of openings and thesubstantially cylindrical outer surface comprises a circumferentialgroove.

In accordance with a fifth aspect, a tubular filter is disclosed thatcomprises expanded metal that has been rolled on itself to form amulti-layered tube having a central bore, a substantially cylindricalouter surface, and substantially flat end sections which extend betweenthe central bore and the substantially cylindrical outer surface,wherein the expanded metal comprises a multiplicity of openings and thecorners formed by the intersections between the substantiallycylindrical outer surface and the substantially flat end sections havebeen rounded by a point loading process.

In accordance with a sixth aspect, a tubular filter is disclosed thatcomprises expanded metal that has been rolled on itself to form amulti-layered tube wherein the expanded metal comprises a multiplicityof openings and the surface of the expanded metal between at least someof the openings is texturized so as to reduce laminar flow of gas overthe surface.

In accordance with a seventh aspect, a tubular filter is disclosed thatcomprises expanded metal that has been rolled on itself to form amulti-layered tube wherein the expanded metal comprises a multiplicityof openings and the expanded metal comprises a first metal at least onesurface of which is coated with a second metal whose thermalconductivity is at least 25% greater than that of the first metal.

Additional features and advantages of the invention are set forth in thedetailed description which follows, and in part will be readily apparentto those skilled in the art from that description or recognized bypracticing the invention as described herein.

It is to be understood that the foregoing general description and thefollowing detailed description are merely exemplary of the invention andare intended to provide an overview or framework for understanding thenature and character of the invention as it is claimed.

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. It is to be understood that the variousaspects and features of the invention disclosed in the specification andin the drawings can be used in any and all combinations. For example,the third through seventh aspects can be used with filters employingexpanded metal sheets which do not have a pitch and/or opening size thatvaries within a single sheet, e.g., these aspects can be used withfilters constructed by welding together individual sheets of expandedmetal, where at least some of the individual sheets differ from oneanother in pitch and/or opening size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of an embodiment of making an expandedmetal sheet and a filter from the sheet.

FIG. 2 is an end view of a filter according to an embodiment of thedisclosure.

FIG. 3A is a sheet having openings on one portion and lacking openingson another, and FIGS. 3B and 3C are tubes formed by rolling the sheet inFIG. 3A.

FIG. 4A is a rolled up expanded metal sheet having two bands ofperforations, and FIG. 4B is the article in FIG. 4A with wound wire.

FIG. 5 is a photograph of an expanded metal filter constructed inaccordance with certain aspects of the disclosure.

FIG. 6 is a photograph of an airbag inflator housing with which theexpanded metal filters disclosed herein can be used.

FIG. 7 is a photograph showing the internal chamber of the housing ofFIG. 6 after deployment of the inflator's explosive.

FIG. 8 is a photograph showing the internal chamber of the housing ofFIG. 6 and the surface of the filter after deployment of the inflator'sexplosive.

FIG. 9 is a schematic drawing illustrating representative features of anexpanded metal strip having openings whose pitch and/or sizes vary alongthe length of the strip. This drawing is not to scale and the number ofsections actually used can be greater or less than that shown in thefigure.

FIG. 10A is a schematic drawing illustrating representative features ofan expanded metal strip having openings whose pitch and/or sizes varyalong the length of the strip. FIG. 10A is not to scale and does notshow all of the sections shown in FIGS. 10B and 10D.

FIG. 10B is a schematic, cross-sectional view of a filter designed toproduce a tortuous path for gases.

FIG. 10C is an enlarged cross-section view of the portion of the filterof FIG. 10B within circle C.

FIG. 10D is a cross-sectional view of the filter of FIG. 10B in a planeorthogonal to the plane of FIG. 10B.

FIG. 11A is a schematic cross-section view of equipment suitable forproducing a pair of grooves in a strip of metal.

FIG. 11B is an enlarged cross-section view of the portion of theequipment of FIG. 11A within circle B.

FIG. 12A is a schematic cross-section view of a filter and a portion ofa housing for the filter where the filter has chamfered corners.

FIG. 12B is an enlarged cross-section view of the portion of the filterand housing of FIG. 12A within circle B.

FIG. 13A is a schematic cross-section view of a filter and a portion ofa housing for the filter where the filter has rounded corners.

FIG. 13B is an enlarged cross-section view of the portion of the filterand housing of FIG. 13A within circle B.

FIGS. 14A and 14B are schematic top and side views, respectively,illustrating the formation of rounded corners on a filter using a pointloading process.

FIG. 15 is a photograph of the surface of a roller for use in forming ametal strip having a texturized surface.

The reference numbers used in the figures correspond to the following:

-   -   3 groove    -   5 expanded metal sheet    -   7 variable section of expanded metal sheet    -   9 non-perforated section of expanded metal sheet    -   13 tubular filter    -   15 airbag inflator housing    -   17 aperture of airbag inflator housing    -   19 rounded corner of airbag inflator housing    -   21 slag captured by filter    -   23 igniter port of airbag inflator housing    -   25 substantially cylindrical outer surface of filter    -   27 rounded corner of filter    -   29 chamfered corner of filter    -   31 substantially flat end section of filter    -   33 point loading roller    -   35 groove of point loading roller    -   37 texturization pattern    -   39 male roller    -   41 female roller    -   43 guide    -   45 protrusion    -   47 recess    -   101 roll of metal strip or sheet    -   103 press    -   105 punch    -   107 teeth or bits    -   109 stretcher    -   111 camera    -   113 computer controller    -   115 monitor    -   121 rollers    -   123 cutter    -   125 expanded metal sheet    -   127 expanded metal sheet    -   129 welder    -   131 cylinder    -   133 welder    -   135 welded mesh cylinder    -   137 female mold    -   139 mandrel    -   201 expanded metal sheet    -   203 expanded metal sheet    -   205 weld    -   207 fabric    -   209 metal screen    -   211 weld    -   301 expanded metal sheet    -   303 perforated portion of expanded metal sheet    -   305 non-perforated area of expanded metal sheet    -   307 area on which wire is wound    -   309 wire winding

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A. Expanded MetalFilters and Methods for Making Such Filters

The following discussion is primarily in terms of filters for automotive(vehicle) airbag inflators (also known as “pyrotechnic airbaginflators,” “slag filters,” or “coolants”), it being understood that themethods and apparatus disclosed herein are also applicable to othertypes of filters such as filters for oil, air, and other liquids andgases, including cleanable filters for these applications. As automotiveairbag inflators are known in the art, details are omitted so as to notobscure the description of the example embodiments.

As discussed above, in certain aspects, this disclosure is for (i)filters made from variable pitch and/or variable opening expanded metalsheets and (ii) methods for making such filters. The sheets can havedifferent spacings between adjacent rows of openings and can, forexample, be made by varying the rate at which the sheet is fed throughthe manufacturing equipment, the rate of punching, and/or the amount ofstretching. In particular, in one embodiment, the disclosure provides anexpanded metal sheet having a multiplicity of rows of openings whereinthe spacing between adjacent rows is not constant.

In one aspect, the disclosure provides an expanded metal sheet wherein arow of perforations is indexed perpendicularly with respect to thelength of the sheet and with respect to at least one other row ofperforations. In another aspect, an expanded metal sheet is providedthat has been flattened to eliminate protrusions (and depressions)caused by the perforation process and to produce a sheet having smoothersides. For many applications, such flattening is not required but it canbe useful in connection with, for example, calibrating opening sizes.

In terms of production methods, in yet another embodiment, thedisclosure provides a method for making a sheet having variably spacedperforations by indexing the punching tool perpendicularly to the(longitudinal) direction of travel of the sheet. This indexing is, ofcourse, in addition to the conventional indexing used in making expandedmetal products in which the perforations in a given row are located atthe midpoints of the perforations of the preceding row. In anotherembodiment the disclosure provides a method for making an expanded metalsheet by (a) feeding a length of metal sheet by discrete steps along thelength of the sheet, (b) perforating to form a row of perforations toproduce a perforated sheet, and (c) stretching the perforated sheetlongitudinally causing the perforations to elongate into openings andproduce an expanded metal sheet, wherein the feeding results in avariable spacing between the rows of perforations.

The disclosure also provides an automobile airbag inflator filtercomprising an expanded metal sheet having a multiplicity of rows ofopenings, the sheet rolled into a cylinder, and secured with a weld, thespacing between adjacent rows of openings varying between differentadjacent rows.

Nesting in a metal filter is reduced by using a variable pitch expandedmetal sheet having different spacings between adjacent rows of openings.Such a filter is made by varying the rate at which the sheet is fedthrough the equipment, the rate of punching, the amount of stretching,and/or the amount by the which sheet is optionally flattened. Nestingcan also be reduced by transversely indexing the punch, using differentpunch sizes on multiple punches, or combinations of these techniques.The filter can be wire wrapped to enhance its strength.

With reference to FIG. 1, the manufacture of an expanded metal sheetaccording to certain aspects of this disclosure starts with a roll ofmetal strip or sheet 101 (for example, about nine inches wide, which canbe cut down to six inches for making filters for passenger airbags andto about 1.5 inches for driver airbags, although any width can be useddepending on the equipment). For filters for airbag inflators, stainlesssteel, such as SS304, 309, 310, 409, 410, or 430 can be used. Carbonsteel from C1006 to C1008 is often preferred for various applications.Also, as discussed below, carbon steel coated with a second metal havinga higher conductivity, e.g., tin, can be advantageous for someapplications. Depending on the environment in which the expanded metalis used, other metal compositions available in a sheet form can be used.

The sheet is fed first to a press 103 in which a punch 105 having anumber of teeth or bits 107 is moved into the sheet so that the teethperforate the sheet and then the punch is removed, just as in a stampingoperation. The geometry of the bits, which are preferably identical toeach other, is preferably such that a slit is formed in the sheet.Depending on the geometry of the bit, the depth of penetration of thebit will determine the length of the slit formed; the deeper thepenetration, the longer the slit, and thus the more open the finalstructure can be after stretching. For airbag inflator filters, theopening is made to a size based on the airbag manufacturer'sspecifications for the open area of the sheet, the porosity of thesheet, or other parameter(s) required for the filter.

The sheet is advanced preferably by a servo motor (not shown) or othermechanism whereby the longitudinal advance of the sheet can be preciselycontrolled. The advance of the sheet is preferably in discrete steps sothat the sheet is stationary when punched. Although not preferred, aroller with teeth can be used in a continuously moved sheet.

The perforated sheet produced in the press is then fed to a stretcher109 in which differential rollers stretch the perforated sheet in theaxial direction (that is, along the direction of travel) so that theslits are opened into diamond-shaped holes. (Of course, a hexagonal bitcan be used to make hexagonal openings, or other bit geometries, can beused, but slits formed into diamonds is the most common shape.)

Although slitting and stretching can be performed as separateoperations, when fine patterns are to be formed, it is often preferableto produce the expanded metal sheets by performing slitting andstretching with the same teeth in the same motion. During thisoperation, the material hangs out over a flattened bottom blade andangled upper teeth or bits slit the sheet and then continue into thesheet. The sheet bends down and the angle formed by this bending as itrelates to the teeth causes a stretching motion of the strip.Consequently, the strip is stretched more or less by the depth of thetooth penetrations. The amount of stretching achieved in this way istypically in the range of 20-25% and can be as much as 37%. Compared tothe slit-and-stretch approach, the one step approach producesperforations (openings) that have a shape more like that of a trianglethan a diamond. As with the separate slitting and stretching approach,the one step approach forms openings by (i) forming slits in a sheet ofmetal and (ii) stretching the slits in the direction of the metal'slongitudinal axis, but does so in one step, rather than two.

A video control system including a camera 111, which is connected with acomputer controller 113 running software, and an optional monitor 115,examines the holes or open area, and can learn (after parameters areinput to the controller) whether the perforations are withinspecification. Although not shown, the camera preferably is movablymounted on a track and is made to traverse the sheet as the sheet isadvanced. The camera preferably captures an entire row (transversely tothe direction of travel) and more preferably a few adjacent rows withinits field of view. The software checks the opening sizes and/or shapes(geometry) to determine whether the individual openings, or open area(actual or estimated or calculated), are within specification. A secondcamera can be placed between the punch and the stretcher with similarhardware and software to determine whether the initial punching iswithin specification. The calculation for determining whether theproduct is within spec is typically based on the light transmittedthrough the openings made by the punch. (Suitable software iscommercially available from Media Cybernetics, Inc., Silver Spring, Md.,under the IMAGE PRO brand.)

While shown with a single punch, multiple punches can be used to providedifferent perforation spacings, geometries, and/or depths. Two punches,for example, can be cycled in any desired order any number of strokes orcycles (such as the first punch alternating with the second, or punchingtwice as often, or half as often, or alternating by twos, and so on).Preferably, at least one press is indexed transversely back and forth sothat adjacent rows made by that press are offset from each other. (Asused herein, a “row” of perforations is preferably transverse to thelength of the sheet, although it is possible to have the press angledwith regard to the longitudinal direction of the sheet.)

The video control system performs an optical inspection of the expandedmetal sheet product and determines whether the product is withinspecification. To alter the process to get on, return to, or change thespecification, the advance of the sheet is altered by adjusting theservo motor (via the computer controller) to change the longitudinalspacing of the perforations. Alternatively, the stretcher is adjusted toincrease or decrease the amount the perforated sheet is stretched. Bothcan be adjusted to further avoid nesting of adjacent layers and/oralignment of radially adjacent openings when the sheet is curled.

One method of providing a variable pitch is to gradually change theadvancement of the sheet. For example, the spacing between rows can begradually increased and then returned to the original value (e.g., witha 500 mil initial spacing, increasing by 1 mil up to 15 mils and thenback to the initial spacing, like a saw tooth waveform; or the changecan be sinusoidal or like a triangular waveform; or irregular). Byhaving a variable pitch, the possibility of nesting can be greatlydecreased. Another method of providing a variable pitch is to have thepitch vary as a function of the circumference defined by a given portionof the sheet in the rolled up filter, so that radially adjacent openingsdo not nest. This approach can be combined with the variable pitchapproach by gradually making the change in pitch between areas on theexpanded sheet corresponding to two radially-adjacent circumferentiallayers, e.g., the change can be made over from two to five press cycles.In this way, overstretching of the transition area between the twopitches can be avoided. For most transitions, overstretching is not aproblem and thus a gradual variation in the pitch is not required.

The expanded metal sheet can be flattened by one or a pair 121 ofrollers. The sheet need not be compressed to any significant degree, andpreferably is not compressed to the extent that flattening would thentend to close up the openings. The video control system camera (or asecond or third camera) can be located after the flattening step, inwhich case it should be appreciated that adjusting the degree offlattening, and the resulting closing of the openings, is an additionalparameter than can be adjusted to achieve the desired open area.Flattening the burrs accomplishes two objectives. The burrs presentareas of high frictional contact; the flattened expanded metal sheet canslide more easily against itself if rolled or curled. The increasedsurface area resulting from flattening allows for increased weldingcurrents and a higher weld strength due to the increased area ofcontact. In addition to these benefits, as noted above, the flatteningsizes the openings. The flattening per se does not substantiallycontribute to anti-nesting and thus for most applications, to avoidunnecessary cost, this additional processing step will not be used.

In the manufacture of a filter for automotive airbag inflators, onefilter geometry is a cylinder having porous walls. To make such adevice, and continuing with FIG. 1, the flattened expanded metal sheetis cut 123 into individual pieces 125 that can be placed overlappinganother (flattened) expanded metal sheet 127, possibly having adifferent open area, and attached to each other via a welder 129(preferably by electric welding). The joined composite sheet is thenrolled into a cylinder 131 and the edge of the mesh secured to thecylinder by a welder 133. To produce the proper ID and OD (inner andouter diameters), the welded mesh cylinder 135 is placed into a femalemold 137 optionally having a movable inner wall, and a mandrel 139optionally expandable is inserted into the central bore of the cylinder.The desired ID and OD of the final filter is achieved by the combinationof the mandrel, optionally expanding, and the mold, optionallycontracting, to cold form the cylinder into the desired radial geometryand dimensions. Because this approach involves a substantial amount ofwelding of individual pieces, flattening can be of benefit since, asdiscussed above, stronger welds can be achieved when welding togethertwo flattened pieces of expanded metal as opposed to two non-flattenedpieces. Another option for the piece-welding approach is to use aconstant pitch and simply rotate selected pieces (e.g., every otherpiece, every second piece, etc.) by 90° before the pieces are weldedtogether. Again, because this approach involves substantial welding,flattening can be of benefit.

In the manufacture of these filters, formed from multiple wrappings ofthe flattened expanded metal sheet, no nesting of the perforations wasobserved. Reduced nesting and the elimination of burrs allows for morefilter layers in a given radial distance. Accordingly, if the designrequires a specified OD, a larger ID can be provided; and likewise aspecified ID will result in a smaller OD and thus a smaller device intotal. Nesting is also deleterious because the shift in alignment of thesheet due to nesting of radially adjacent openings may not render thefilter end (cylinder top and/or bottom) dimensions out of spec, whilenevertheless providing an open channel between adjacent filter layers.Even if openings in adjacent layers are almost aligned, the absence ofburrs eliminates the tendency for the openings to align (the burr of oneopening settling into the adjacent opening) and thus decrease thefiltering capacity.

The weld strength of the flattened sheet is twice that achieved when thesheet was not flattened. Spot welding is typically automated through amachine in which the welding current is operator-adjusted. Setting afixed current leads to inconsistent welds when a non-flattened expandedmetal sheet is welded because the burr areas are not uniform and surfacearea of contact through which the weld current flows varies for eachweld. When the expanded metal sheet is flattened, there is a largersurface area, and so a larger welding current can be used. It was foundthat doubling the welding current for welding a flattened expanded metalsheet resulted in a weld strength more than double the weld strength ofa sheet with burrs, as well as a more consistent weld strength.

The filter described above is made from two expanded metal sheets rolledinto a cylinder. During the rolling, one or more intermediate layers canbe added so that the filter has multiple repeating layers, or differentintermediate layers each at a different radial distance. For example,FIG. 2 depicts the end (or cross-section) of a rolled cylindrical filterwherein a first flattened expanded metal sheet 201 is joined to a secondflattened expanded metal sheet 203 by a weld 205 and rolling is startedfrom the first sheet. At a predetermined position or amount of rolling,a fabric 207 is inserted between the layers, and at a position radiallyoutward from the predetermined position a metal screen 209 is insertedbetween the layers of metal sheet. The outermost layer of the metalsheet is attached to itself by a weld 211. The filter thus formed hasmultiple filtering layers of different materials.

FIG. 3A depicts an expanded metal sheet 301 having a portion 303 thathas been perforated and expanded, and disposed between areas 305 thathave not been perforated. When punching is resumed after a solid(unperforated) area it preferably begins gradually, with the first fewpunches preferably not perforating the sheet to avoid the tendency ofthe punch to cut the sheet in two when a solid area is presentdownstream of the area to be perforated. The sheet has long edges Lformed by the original sheet, and short edges S where it has been cut asa portion from a larger sheet (such as shown in FIG. 1). Some airbagdesigns require the flow of the expanded gas (whether by explosion or acompressed gas, or combination thereof) to be directed in a particulardirection. For example, a curtain-type airbag may require gas to bedirected along a linear extent. Alternatively, the area in which theairbag assembly is installed may have only a portion available for fluidcommunication with the airbag. The sheet in FIG. 3A can be rolled up,using as its axis a line connecting the short sides. In this case, thelong sides will be overlapped and welded to create a filter having thegeometry shown in FIG. 3B, where the ends of the filter are unperforatedand only the central section is perforated. Alternatively, the sheet canbe rolled using as the axis a line connecting the long sides, whichresults in the geometry shown in FIG. 3C. Although sheet 301 is shown ashaving a single perforated section, multiple perforated sections,separated by unperforated sections, can be made (by not punching overthose areas) to create a filter as shown in FIG. 3B but having twoperforated bands separated by an unperforated area, as shown in FIG. 4A,as opposed to the single perforated band shown. It is apparent in viewof the foregoing examples that various portions can be perforated orleft unperforated, and the sheet cut and rolled to provide a tubularfilter having perforations only at predetermined areas to direct the gasflow as desired.

The pattern of openings is determined by the arrangement of bits on thepunch and the punch rate as a function of the linear travel of thesheet. Avoiding nesting can be accomplished by various modifications ofthis basic design. As noted above, multiple punches can be used havingdifferent arrangements of bits (for example, laterally offset from theother punch(es)). The rate of punching can be altered to provide anincreasing distance between adjacent openings (the pitch), and thencycled back, as noted above. For instance, the travel of the sheet canbe adjusted so that the pitch increases from the previously punched rowby one mil until the separation is 15 mils, and then the process isreversed (decreasing each by one mil) or started from the beginningagain. Given the specifications for a particular filter, the pitch canbe varied as a function of the circumference defined by a given portionof the sheet in the rolled up filter, so that radially adjacent openingsdo not nest. Yet another method for preventing nesting is by indexingone or more punches transversely so that each row is offset laterallyfrom the previous row. As with the other methods, the punch can beindexed back and forth by a fixed amount, or by a predetermined amountone way until a desired offset is achieved, and then back. For example,using the center-to-center distance (for the openings) in a given row,the next row can be indexed transversely 30% on the next punch, anadditional 30% on the following punch, and so on, for a predeterminednumber of times, and then indexed back to the original position. Knowingthe position of any portion of the sheet in the filter, as based on thefilter specification, allows a straightforward calculation to determinehow the openings need to be varied to avoid nesting radially adjacentopenings. Any one or combination of these techniques can be used toreduce the possibility of nesting, including those described below.

Still another method to reduce nesting is to vary the size of theopenings. Preferably, the size of the openings in any row can be variedby altering the punch depth, such as by varying the stop position of thepunch every one, two, or desired number of punches. When two or morepunches are used, each can be set at a different punch depth, and/orhave bits of a different size than another punch, and/or have bits of adifferent geometry.

The expanded metal articles of this invention can be combined with wirewinding. For example, a sheet perforated as desired is rolled onto amandrel so that the ends abut, and then wire is wrapped around thisperforated substrate in a desired pattern. The winding can reinforce thesolid (unperforated) areas, provide a chamfered end, and/or partiallycover the perforations. The wrapping of the wire avoids the need to weldthe edges of the sheet. FIG. 4B shows the device in FIG. 4A having threeseparate areas 307 on which wire is wound. The wire increases thestrength of the device in resisting the explosive force of the gascharge. One end is shown where the winding 309 provides the geometry ofa chamfered end (or approximation of a chamfered end). The wire woundperforated sheet can be sintered or brazed to secure the wire wrapping.When wound over existing perforations, the winding can be used tofurther increase or tailor the effective opening space (that is, theresulting pressure drop) to a desired degree.

Although the above embodiments have been described with particularreference to filters for airbag assemblies, there are other uses forsuch an article, such as an electrode and to replace the Dutch weaveused in woven airbag filters. As a thinner sheet is used in thebeginning, smaller holes are possible, enabling the production of a“micro” expanded metal. Certain prior art perforated metal sheet filtersemployed a ceramic paper interlayer for additional filtering. Instead,the microexpanded metal sheet (thin enough to be considered a foil, lessthan 10 mils in thickness) can be used instead of ceramic paper. In mostcases, the articles produced by this disclosure will be used forfiltering.

B. Applications

In a typical application of the foregoing disclosure, the expanded metalsheet is rolled upon itself to produce a structure having multiplelayers, e.g., between three and twenty layers. The first 360 degree wrap(first layer) can be secured with spot welds with the remaining layersbeing continuously wrapped around one another to reach the desiredoutside diameter. The outermost layer can then be secured with spotwelds. For strength purposes, spot welds can be added to one or more ofthe internal circumferential layers of the filter. Such additional spotwelds, however, are generally not required to achieve the strengthlevels needed for an airbag filter.

FIG. 5 shows an airbag inflator filter 13 constructed in this way. FIG.6 shows an example of an inflator housing 15 having an internal chamberin which filter 13 is inserted. The housing shown is for a steeringwheel airbag and thus has an overall “pancake” shape suitable formounting in a steering column. The housings for other types of airbags,e.g., curtain airbags, have somewhat different configurations and thusthe configuration of filter 13 may be different for such airbags.Although the configuration may be different, the same basic non-nestingconstruction is used.

As shown in FIG. 6, housing 15 includes a plurality of apertures 17which allow gases produced by the inflator's explosive charge to exitthe housing and inflate the air bag (not shown) which is secured aboutthe outside of the housing. A typical explosive charge is based onammonium nitrate and produces substantial amounts of particulate debris,known in the art as “slag.” FIGS. 7 and 8 show the slag residue 21captured on the inner diameter of filter 13 after a charge has beendetonated. Reference number 23 in this figure shows the housing'signiter port which is used in detonating the main explosive charge whichis located within the inside diameter of filter 13 prior to detonation.An examination of FIGS. 7 and 8 gives some feeling for the conditionsunder which filter 13 must operate. Comparing these figures with FIG. 5,which shows the filter prior to detonation, shows the extreme pressureand heat conditions to which the filter is exposed during use.

In constructing filter 13, the opening or pitch or both along the lengthof the expanded metal sheet are normally changed at everycircumferential wrap and the developed pattern shows an increasedsection length as the filter is wound in that the circumferenceincreases at every additional layer of expanded metal. FIG. 9schematically illustrates representative types of patterning that can beused along the length of expanded metal sheet 5, where reference number7 represents a section of the expanded metal sheet which has a pitchand/or opening size selected to reduce nesting, and reference number 9represents a non-perforated section (see discussion below). In additionto considerations of nesting, the hole sizes can also be selected basedon desired gas flow behavior. Thus, larger openings can be used for theinnermost layers of the filter, followed by finer, but non-clogging,openings, followed again by larger openings to achieve a diffusingeffect at the outer surface of the filter. Other combinations can, ofcourse, be used based on the specifics of the application.

Lengths L1, L2, and L3 in FIG. 9 (as well as the lengths of the othersections shown in this figure and in FIG. 10A) will in general bedifferent so as to take into account the increases in circumferentiallength associated with increasing radius. It should be noted that thesection lengths in this figure and FIG. 10A are not drawn to scale, butare merely illustrative of the types of structures used to reducenesting of the circumferential wraps. Also, the number of sections usedin an actual filter can be more or less than the number shown in thesefigures.

By using the variable pitch and/or opening sizes, a variety of benefitscan be achieved. For example, by using a fine pitch and/or a fineopening size, particle retention and overall performance can beoptimized. More generally, by reducing/eliminating nesting, consistencyis greatly improved, including substantial reductions in the standarddeviation for flow through the filter, which is of primary importance tomanufacturers of air bag systems.

C. Torturous Paths

FIG. 10 illustrates the use of non-perforated sections 9 to produce atorturous path for gas flow through the filter. These non-perforatedsections or stripes in the perforation pattern result in a zigzag gaspath in the assembled filter. Such a path is desirable for trappingparticles generated by an airbag's explosive charge. In particular, eachtime the gas is forced to change direction, its velocity profile changesand this causes particles entrained in the gas stream to plate out andbecome trapped in the filter. One of the primary purposes of air baginflator filters is to capture particles produced by the explosivecharge and thus improvements in the particle capturing ability offilters is of importance to manufacturers of air bag systems.

Non-perforated sections 9 produce such changes in direction since thegas cannot pass through these sections but must travel along the sectionuntil it reaches the section's edge where it again must turn to find apath out of the filter. The non-perforated sections thus act as barriersto simple radial flow of gases through the filter.

This zigzagging is illustrated in FIG. 10, where FIG. 10C is an enlargedview of the portion of filter 13 within circle C of FIG. 10B. The arrowsin FIG. 10C represent gas flow through the filter. As is evident, thegas flow path is highly complex with numerous changes in direction whichassist in removing particles from the gas stream.

In FIG. 10, non-perforated sections 9 extend across the entire width ofexpanded metal sheet 5. If desired, the non-perforated sections canextend only partially across the width of the sheet, e.g., there can bea perforated region at one edge, at both edges, at one or more locationsbetween the edges, or combinations thereof. For some applications, suchmore complex patterns can be helpful in balancing the need for particleretention with the need for low backpressure.

However deployed, the use of non-perforated sections causes the gas flowto include at least some circumferential flow within the body of thefilter. The circumferential flow produced by the non-perforated sectionsis over distances greater than the average spacing between perforations,e.g., more than 5 times greater. In some embodiments, e.g., theembodiment illustrated in FIGS. 10B and 10D, substantially all possibleflow paths from the inner diameter of the filter to its outer diameterincludes both radial and at least some circumferential flow over adistance greater than the average distance between perforations.

It should be noted that not every layer of the filter need havenon-perforated sections. Rather, some layers can exhibit primarilyradial flow, e.g., the first few and last few layers, and others can bea combination of radial and substantial circumferential flow, e.g., themiddle layers.

D. Circumferential Grooves

As illustrated in FIG. 6, housing 15 includes a plurality of apertures17 through which the gases generated by the airbag's explosive chargeexit the housing. Typically, these apertures are in a single transverseplane, although in some cases two rows of apertures in two transverseplanes are used. As illustrated in FIGS. 6-8, the apertures, whether inone or more than one transverse plane, are typically not symmetricallylocated with respect to the housing's internal chamber which receivesfilter 13.

Because they are the only exit for the gases produced by the airbag'sexplosive charge, it is important that apertures 17 remain openthroughout the airbag deployment. In practice, filters 13 and, inparticular, the outer wraps of the filter, can expand as a result of theexplosion taking place within the filter's inner diameter. Suchexpansion can reduce and/or close off the annular radial manifold spacebetween the filter and the housing, including the space in the vicinityof apertures 17, thereby causing increased combustion pressure.

Through a series of experiments, it was found that a groove formed inthe outer surface of the filter will substantially retain its shapeduring the explosion, notwithstanding the fact that the rest of thefilter grows in size. These experiments included experiments underextreme conditions, i.e., experiments employing a maximum explosivecharge and a filter temperature of 250° F. The 250° F. temperature wasselected since it represents the maximum temperature likely to occur ifa car is left in the sun for a substantial period of time. Testing underelevated temperature conditions was performed since the physicalproperties of metals, such as those used to form filter 13, degrade withincreasing temperature.

Based on these experiments, it was concluded that the problem of reducedaccess to apertures 17 as a result of deformation of the filter by thepressures generated during detonation could be overcome by (i) includinga circumferential groove 3 in the outer substantially cylindricalsurface of the filter and (ii) ensuring that in the assembled housing,the groove is aligned with the circumferential ring of apertures 17.

As noted above, apertures 17 are typically offset from the midplane ofthe chamber in which the filter is received. Accordingly, if the filterwere to have just one groove and if the filter were to be inserted inthe cavity in the wrong orientation, access to apertures 17 could becomecompromised. To address this possibility, two grooves 3 are formed inthe outer surface of filter 13 so that one of the grooves is alignedwith the housing's apertures irrespective of the filter's orientationwhen inserted into the housing. As also noted above, some housings havemore than one row of apertures. In such a case, multiple pairs ofgrooves 3 are formed in the outer surface of filter 13, with one groovebeing aligned with each row of apertures irrespective of the orientationof the filter when inserted in the housing. For example, if the housinghas two closely spaced rows of apertures, the filter would have fourgrooves organized into two groups, with the spacing between the groovesof each group being smaller than the spacing between the groups.

Groove 3 can have various configurations and can be produced in variousways. In general terms, the groove should be wide enough so that it isover the apertures even if all of the tolerances, i.e., the accumulatedor stacked tolerances, happen to be in one direction. In practice,grooves having arcuate cross sections have been found to resist higherpressures than those having V-shaped cross sections. Also, forminggrooves in the expanded metal sheet prior to rolling the sheet on itselfto form the filter has been found to produce a groove that is betterable to withstand the detonation pressures than grooves formed in theoutside surface of the filter after the rolling has been completed.

In addition to using grooves, filter expansion can also be reduced byincreasing the tensile strength of the metal used to form the filter.The increased tensile strength increases the filter's hoop strength,thus minimizing the growth of the outer wraps of the filter.

FIG. 11 shows equipment that can be used to form grooves 3 in anexpanded metal sheet as it is being wound into a filter. As showntherein, a first roller 41 includes recesses 47 and a second roller 39includes protrusions 45. The expanded metal sheet is passed between therollers with guides 43 serving to restrain the sheet from sidewisemotion. Guides 43 also maintain the alignment between the rollers, e.g.,roller 39 can be allowed to float in a transverse direction with guides43 being used to trap the roller and align it with roller 41.Alternatively, a single pressure roller with the groove configurationcan be forced pneumatically, hydraulically, or with a servo motor ontothe surface of the filter as it is being wound thereby forcing thegroove shape into the filter. As a further alternative, after the filterhas been completely wound and welded, a contoured groove roller can beforced onto the substantially cylindrical face of the filter as it isrotated to form the groove. This latter approach, however, has beenfound to produce grooves which are less able to withstand the forcesassociated with detonation than the approaches in which the grooves areformed in the expanded metal sheet as it is being wound.

E. Rounded Corners

It has also been determined experimentally that applying a radius to thecorners of the filter produces a substantially better seal of the filterin the inflator housing than either a sharp corner or a chamferedcorner, where a better seal means one that reduces the maximum length offlame released from the housing when an inflator charge is ignitedwithin the central bore of the filter. As known in the art, such flamelength can be determined using high speed photography of an airbaghousing during detonation.

As illustrated in FIGS. 6, 12, and 13, inflator housings typically haverounded corners (see reference number 19 in these figures). FIG. 12shows the mating of a filter 13 having a chamfered corner 29 withrounded corner 19, while FIG. 13 shows the mating of a filter having arounded corner 27. Before the experiments were performed, it wasexpected that the chamfered corner would perform better than the roundedcorner because the point contacts associated with a chamfered cornerwere expected to produce deformation of the corner by point loading andthus a custom seal between the filter and the housing's wall as thefilter was forced into the wall as the housing was assembled and then bythe pressure generated by the explosive charge.

In practice, however, it was found that the rounded corner wassubstantially better than the chamfered corner. Thus, switching from thechamfered corner of FIG. 12 to the rounded corner of FIG. 13 produced areduction of more than 25% in the maximum length of flame released fromthe housing when an inflator charge was ignited within the central boreof the filter. The chamfered corner was better than a square corner, butnot as good as the rounded corner. Compared to a square corner, therounded corner produced a reduction in the maximum length of the flamereleased from the housing substantially greater than 25%, e.g., areduction greater than 50%. The rounded corner was also better than boththe chamfered and square corners in particle retention.

In general terms, the mean radius of the filter's corner should bewithin ±10%, preferably, within ±5% of the mean inner radius of thehousing. Also, the rounding of the corner needs to be performed using apoint loading process. In particular, machining the radius is notsuitable because of the low structural strength of the individual filterlayers. Grinding is also not viable because grinding residue, bothmetallic and organic, will be deposited into the filter structure andwill be difficult, if not impossible, to remove. Such grinding residuewill be released when an airbag's explosive charge is detonated and thuswill represent debris not captured by the filter. To form the radius ina single stroke in a coining or forming guide is not practical becausethe column strength of the filter is not adequate to support thesubstantial forces associated with such operations.

A point loading process overcomes the problems with these otherapproaches. Such a process densifies the corner without metal removal,thus avoiding the debris problem associated with grinding. At any giventime, the process only applies high forces to a small area of thecircumference of the filter's edge, thus avoiding the problemsassociated with the strength characteristics of the filter.

The point loading can be performed in a variety of ways. For example, asin an orbital riveting machine, one or more rotating shafts with theappropriate radius on their working surface can be applied to points onthe corner of the filter and moved along the corner's circumference togenerate the rounded corner. Alternatively, a device of the type shownin FIG. 14 can be used.

In this system, both roller 33 and filter 13 rotate, as indicated by thecurved arrows in FIG. 14B. In particular, filter 13 is driven atrelatively high speed and roller 33 is brought into contact with thefilter and rotates in the opposite direction from the filter as a resultof frictional engagement with the filter. Filter 13 fits within aperture35 of roller 33 and as shown most clearly in FIG. 14B, the aperture andthe filter only make contact along the single line where they meet.Accordingly, the force applied between the filter and roller 33 to bringthem into engagement is only applied to the two corners of the filter atthe opposite ends of this line of contact, i.e., the filter's cornersare deformed by point loading. The radii of the corners of aperture 35match those of the housing within ±10% (preferably, ±5%) and thus, asillustrated in FIG. 13B, the result of the point loading is to produce afilter corner 27 between the filter's substantially cylindrical outersurface 25 and its substantially flat end section 31 which closelymatches the inside radius of the housing's corner 19.

F. Texturization

As discussed above, one the purposes of airbag inflator filters is tocapture debris (particles) generated by the detonation of the inflator'sexplosive charge. As also discussed above, particles tend to be caughtby the filter when the gases flowing through the filter are undergoing achange in direction. Laminar flow is the antithesis of a flow in whichgases change direction. To minimize the occurrence of such flow, thesurface of the expanded metal making up the filter can be texturized.Such texturization can be performed on the metal sheet before it isperforated (expanded) or after, with pre-texturization being the moretypical approach.

By texturizing the metal surface, a relatively flat planer surface istransformed into a series of thousands of hills and valleys, thusincreasing the surface area of the sheet by upwards of 20%. Having thisadditional surface area at the boundary over which the hot gases passdramatically increases the filtering efficiency of the filter. Inaddition to improving filtering, a texturized surface increases thermalheat transfer to the filter. As discussed above, cooling the gasesproduced by an airbag's explosive charge before they reach the airbag isone of the function of airbag filters.

Although not wishing to be bound by any particular theory of operation,it is believed that both the increased filtering efficiency and theincreased cooling of the explosive gases is due to the generation ofturbulent flow with high Reynolds numbers at the texturized surface.Such turbulent flow is dominated by internal forces which tend toproduce random eddies, vortices, and other flow fluctuations. Theserandom eddies, vortices, and flow fluctuations keep the gases in contactwith the filter surfaces longer, thereby depositing more particles onto,and exchanging more heat energy with, the filter.

With regard to capturing particles, it is also believed that the peaksand valleys of a texturized surface which are perpendicular to thedirection of gas flow contribute to the additional particle removal and,in particular, to the removal of fine particles. These peaks and valleysprovide a mechanism for tenaciously arresting and removing particlesfrom the gas stream.

Because the gases come into contact with both sides of the metal stripor strips making up the filter, the texturization is preferably on bothsides of the strip(s), although texturization on one side can be used ifdesired. The texturization can be performed in various ways known in theart. One of the more efficient ways is to pass the strip(s) throughembossing rollers having a pattern engraved in their surface. Thepattern on the rollers can be formed in a number of ways known in theart, including by hand, laser etching, EDM, hobbing, or combinationsthereof. As alternatives to the use of embossing rollers, lasers, acidetching, knurling, sandblasting (e.g., with a centrifugal blastingdevice), or combinations thereof can be used to directly texturize thesurfaces of the strip. However formed, the linear feature density D ofthe texturized surface can be in the range of 15-500 features/inch, withthe feature amplitude being no greater than 0.5/D.

FIG. 15 is a photograph of the surface of an embossing roll having atexturization pattern 37 formed therein that can be used to texturizemetal strip(s) used to produce airbag filters. The U.S. penny in thisfigure shows the scale of the types of patterns that can be used. Asshown in this figure, the overall pattern is a checkerboard with thegrooves in each block at right angles to those of adjacent blocks. Whentransferred to a metal strip, a pattern of this type will significantlyreduce the ability of a gas to exhibit laminar flow as it passes overthe strip's surface. The pattern is also able to grab particles from thegas stream even with fluctuating flow paths.

G. Coated Metals (e.g., Plated Metals)

As discussed above, in airbag inflator systems, in addition to capturingparticles, filters are also utilized to cool the expanding hot gasesfrom the pyrotechnic deployment. The cooling ability of the filter has atremendous influence on the performance of the inflator.

By forming the filter out of one or more expanded metal strips thatcomprise a first metal having at least one surface coated with a secondmetal whose conductivity is greater than the first metal, e.g., at least25% greater, additional heat transfer from the expanding gases to thefilter can be achieved beyond that provided by the first metal alone. Asused herein, the word “metal” includes pure metals and metal alloys.

During deployment, thermal heat energy is transferred to the filter byconvection. Because the entire deployment of the inflator's explosivetakes only about 20 milliseconds, the time available for convectivetransfer is short. Accordingly, there is a tremendous advantage inhaving a highly thermally conductive outer layer at the boundary betweenthe filter and the hot gases. Because of its higher thermalconductivity, even a thin layer of the second metal such as thatproduced by plating will enhance both the local transfer of heat to thefilter and the distribution of the heat throughout the body of thefilter.

A variety of metals can be used for the first and second metals. Forexample, the first metal can be a carbon steel and the second metal canbe tin or a tin alloy. Mild steel has a thermal conductivity in therange of ˜26 W/mK to ˜38 W/mK, while pure tin's thermal conductivity is˜64 W/mK. As noted above, an overriding consideration when manufacturingmass-produced items for the automotive industry is cost. Because of itsuse in the canning industry, tin-plated carbon steel sheet is readilyavailable. Although it is somewhat more expensive than non-platedmaterials, the thermal efficiencies accrued by utilizing tin-platedexpanded metal allow the overall mass of the filter to be reduced whichcan more than compensate for the slight increase in cost of thetin-plated carbon steel.

In addition to the foregoing considerations, there is sometimes a needto protect the filter from corrosion prior to the time it is used, whichmay be many years after it is installed. There are two filter locationsused in pyrotechnic inflators. In the first, the filter is inside ahermetically sealed chamber with the propellant. In the second, thefilter is outside the hermetically sealed chamber and thus is exposed tothe atmosphere where it can rust or become corroded. In the past, theproblem of corrosion has been dealt with by using stainless steel toconstruct the filter or by performing a thermal bluing operation on thefinished filter similar to the bluing on firearms. Such bluing forms anoxide on the filter's metal surface which retards rusting.

An advantage of tin-plated carbon steel as it relates to filters madefrom expanded metal sheets is the material's natural corrosionresistance. Although in most cases the steel sheet will be plated beforethe expanding operation takes place, thereby leaving the pierced andexpanded holes somewhat unprotected, the majority of the sheet is stillcovered with the tin plating. Also, experimental studies have shown thatthe piercing tooling drags some of the tin plating into the openingsduring the expanding operation and although the openings do not exhibitthe same corrosion resistance as the surfaces of the sheet there isstill an increase in protection in the openings.

The foregoing description is meant to be illustrative and not limiting.Various changes, modifications, and additions may become apparent to theskilled artisan upon a perusal of this specification, and such are meantto be within the scope and spirit of the invention as defined by theclaims.

For example, although filters that comprise at least one strip ofexpanded metal having a variable perforation pattern along its length(i.e., variations in pitch and/or opening size) are preferred (e.g.,because of their higher consistency in airbag filling and lower levelsof out-of-roundness as formed), various aspects of the disclosure can bepracticed in connection with filters produced by welding together piecesof expanded metal having different perforation patterns which areuniform throughout the length of each piece. In particular and withoutlimitation, the torturous path, circumferential groove, rounded corners,texturization, and coated metal aspects of the disclosure can be used inconnection with this latter approach for making expanded metal filtersand the claims set forth below directed to these aspects of thedisclosure are intended to cover both approaches for making expandedmetal filters, as well as other approaches now known or subsequentlydeveloped.

1-4. (canceled)
 5. A method of making a filter comprising: (a) providinga sheet of metal having a width and an axis; (b) forming slits in thesheet and stretching the slits in the direction of the sheet's axis toform a multiplicity of rows of openings; (c) cutting a smaller sheetfrom the sheet produced in step (b); (d) rolling the smaller sheet ofstep (c) on itself to form a tube; and (e) securing the tube of step (d)with a weld; wherein in step (b), the multiplicity of rows of openingsare arranged to reduce nesting when the smaller sheet is rolled onitself to form the tube.
 6. The method of claim 5 wherein the pitchbetween the rows of openings, the sizes of the openings, or both thepitch between the rows of openings and the sizes of the openings arevaried to reduce nesting when the smaller sheet is rolled on itself toform the tube.
 7. The method of claim 6 wherein the pitch between rowsof openings is varied as a function of the circumference defined by agiven portion of the rolled up smaller sheet so that radially adjacentopenings do not nest.
 8. The method of claim 6 wherein in step (b) thesheet is fed at a rate which varies so as to vary the pitch between therows of openings.
 9. The method of claim 5 wherein the smaller sheetincludes areas that do not include openings.
 10. The method of claim 5wherein the filter is an automotive airbag filter. 11-28. (canceled) 29.A method of making a filter comprising: (a) providing a sheet ofexpanded metal having a multiplicity of openings; (b) cutting a smallersheet from the sheet of step (a); (c) rolling the smaller sheet of step(b) on itself to form a tube having multiple layers; and (d) securingthe tube of step (c) with a weld; wherein the openings are arranged inrows and, for adjacent layers of the rolled up filter, the pitches ofthe rows are different.
 30. The method of claim 29 wherein the pitchesare varied as a function of the circumference defined by a given portionof the expanded metal in the rolled up filter so as to reduce nesting ofradially adjacent openings.
 31. The method of claim 29 wherein thesmaller sheet includes areas that do not include openings.
 32. Themethod of claim 29 wherein the smaller sheet has long sides and shortsides and in step (c), the smaller sheet is rolled on itself about anaxis connecting the long sides.
 33. The method of claim 29 wherein thesmaller sheet has long sides and short sides and in step (c), thesmaller sheet is rolled on itself about an axis connecting the shortsides.
 34. The method of claim 29 comprising the further step of usingthe filter for an inflator of an airbag device.
 35. A method of making afilter comprising: (a) providing a sheet of expanded metal having amultiplicity of openings; (b) cutting a smaller sheet from the sheet of(a); (c) rolling the smaller sheet of step (b) on itself to form a tubehaving multiple layers; and (d) securing the tube of step (c) with aweld; wherein the openings are not all of the same size and the openingsare arranged so that for adjacent layers in the rolled up filter, thesizes of the openings are different.
 36. The method of claim 35 whereinthe sizes are varied so as to reduce nesting of radially adjacentopenings in the rolled up filter.
 37. The method of claim 35 wherein thesmaller sheet includes areas that do not include openings.
 38. Themethod of claim 35 wherein the smaller sheet has long sides and shortsides and in step (c), the smaller sheet is rolled on itself about anaxis connecting the long sides.
 39. The method of claim 35 wherein thesmaller sheet has long sides and short sides and in step (c), thesmaller sheet is rolled on itself about an axis connecting the shortsides.
 40. The method of claim 35 comprising the further step of usingthe filter for an inflator of an airbag device.
 41. A method of making afilter comprising: (a) providing a sheet of expanded metal having amultiplicity of openings; (b) cutting a smaller sheet from the sheetproduced in step (a); (c) rolling the smaller sheet of step (b) onitself to form a tube having multiple layers; and (d) securing the tubeof step (c) with a weld; wherein the openings are not all of the sameshape and the openings are arranged so that for adjacent layers in therolled up filter, the shapes of the openings are different.
 42. Themethod of claim 41 wherein the shapes are varied so as to reduce nestingof radially adjacent openings in the rolled up filter.
 43. The method ofclaim 41 wherein the sizes are varied so as to reduce nesting ofradially adjacent openings in the rolled up filter.
 44. The method ofclaim 41 wherein the smaller sheet includes areas that do not includeopenings.
 45. The method of claim 41 wherein the smaller sheet has longsides and short sides and in step (c), the smaller sheet is rolled onitself about an axis connecting the long sides.
 46. The method of claim41 wherein the smaller sheet has long sides and short sides and in step(c), the smaller sheet is rolled on itself about an axis connecting theshort sides.
 47. The method of claim 41 comprising the further step ofusing the filter for an inflator of an airbag device.